1. Field of the Invention
The present invention relates to a pixel structure in a liquid crystal display device, such as a liquid crystal projector, and more particularly to improvements in light-shielding capability of an active matrix type liquid crystal display device for a light valve wherein switching of a liquid crystal is carried out through a thin film transistor (TFT). Further, the present invention relates to a manufacturing method thereof.
2. Description of the Related Art
In recent years, as a display for the wall-hanging type television, the projective-type television or the OA appliances, various display units using a liquid crystal panel have been developed. Among those liquid crystal panels, an active matrix type liquid crystal display wherein TFTs are included as the active element in a liquid crystal display device is the most promising to realize a high quality display unit for the OA appliances as well as a display unit for the high definition television because of its advantageous natures such as the one that an increase in number of scanning lines therein does not result in a deterioration of the contrast or the response time thereof. Especially when applied to a projection type liquid crystal display with the liquid crystal projection or the like, it allows achieving a large screen display with ease.
Normally, in the active matrix type liquid crystal display device for a light valve that is utilized for the liquid crystal projection, a small element is irradiated with a strong light, and the light passing therethrough is controlled according to the image data by turning on and off each pixel separately through switching of a liquid crystal by a corresponding TFT, and then the transmitted light, being magnified by an optical element such as a lens or the like, is projected on a screen or the like. At this, if an active layer of the TFTs formed from polysilicon (p-si), there may be produced the leakage current in a channel section of the TFT at the off-time, due to the photo excitation, by the reflected light from the optical system such as a lens, not to mention under the direct influence of the incident light thereon, and brought about a problem of display quality such as cross-talk.
The present assignee have so far put forward several methods of preventing the light from entering into the channel section of the TFT. For example,in Japanese Patent Application Laid-open No. 80476/1997, it is disclosed that, by setting a light-shielding film beneath a TFT, that is, on the opposite side to the incident light from the light source, a reflected ray from the optical system can be prevented from entering into the TFT, while, by forming a section of rough surface on the substrate surface over which the light-shielding film is formed, a reflected ray entering through a section where no light-shielding film is formed can be made to make diffused reflection within the substrate and, thus, prevented from entering into the TFT.
Further, in Japanese Patent Application Laid-open No. 164875/2000, there is described a structure wherein a recess section is formed in a substrate, and, in the recess section, a light-shielding film is formed, and, within that recess section covered with the light-shielding film, a channel section of a TFT is formed, whereby the reflected light can be prevented from entering thereto.
FIG. 25(a) is a schematic partial plan view and FIG. 25(b) is a schematic cross-sectional view taken along the line G-Gxe2x80x2 of FIG. 25(a) for a liquid crystal display device in process of manufacture according to Japanese Patent Application Laid-open No. 164875/2000.
In the drawings, referential numeral 1 indicates a transparent insulating substrate such as a glass substrate, and on an inner wall of a recess section (trench) set within an underlying insulating film 2 on the substrate, an underneath light-shielding film 4 is formed from WSi or the like. Covering the underneath light-shielding film 4, a first interlayer film 5 is formed over the substrate, and, in a section surrounded with the afore-mentioned underneath light-shielding film 4, a semiconductor layer 7 that is to serve as a channel layer of a TFT is formed from polysilicon, and over that semiconductor layer 7, a gate insulating film 8 and a gate line 9 are successively formed. Such a structure can considerably cut off the incidence of the reflected light from the optical system and improve the display quality. Further, this structure may reduce, with effect, the unevenness resulting from the overlapping of the layered films.
However, the first interlayer film 5 that is to be formed on the underneath light-shielding film 4 is set to have a considerably large film thickness, for the purpose of protecting the semiconductor layer 7 made of polysilicon from contamination and besides, if a conductive material such as WSi is utilized for the underneath light-shielding film 4, with an object of preventing the underneath light-shielding film to act as a back-gate. In consequence, as shown in the drawing, the light, although in a small amount, may enter from a gap between the underneath light-shielding film 4 and the gate line 9, and then this incident light may reach the semiconductor layer 7, making diffused reflection within the first interlayer film 5 and give rise to the leakage current, which causes the lowering of the image quality.
Accordingly, an object of the present invention is to provide an active matrix type liquid crystal display device having a structure in which a pixel TFT is disposed in a trench set in a substrate thereof with an arrangement to prevent an incident ray of light which enters from an edge section of an underneath light-shielding film from reaching a semiconductor layer of the pixel TFT.
In light of the above problems, the present inventors conducted investigations and reached the present invention by finding out that the light can be blocked from traveling in for certain, if, when forming a trench in the afore-mentioned substrate, a hill-shaped section is left in the vicinity of the pixel TFT, without being carved in, and the underneath light-shielding film extends to the top of said hill-shaped section, and a portion of the interlayer film formed on said underneath light-shielding film, which lies on said hill-shaped section, is made thin, and thereby a distance between a metal interconnection layer such as a gate line that is to be formed in the later steps and the underneath light-shielding film is made substantially narrow.
Accordingly, the present invention relates to
(1) an active matrix type liquid crystal display device having a structure in which a pixel TFT is disposed in a trench carved in a substrate; wherein
with a section which is not carved in but left hill-shaped being present in the vicinity of the TFT, an underneath light-shielding film disposed beneath a semiconductor layer of the TFT is formed so as to reach at least the top of said hill-shaped section; and a metal electrode layer formed above the semiconductor layer of the TFT extended to the top of said hill-shaped section; and besides, on the top of said hill-shaped section, a film thickness of an interlayer insulating film laid between said underneath light-shielding film and metal electrode layer is made thinner than in other sections thereof.
Further, the present invention relates to
(2) a liquid crystal display device as set forth in (1), wherein the interlayer insulating film laid between said underneath light-shielding film and metal electrode layer comprises a first interlayer film formed between the underneath light-shielding film and the semiconductor layer as well as a gate insulating film formed between the semiconductor layer and the metal electrode layer; and, on the top of said hill-shaped section, at least a part of said first interlayer film in the direction of the thickness is etched away.
Further, the present invention relates to
(3) a liquid crystal display device as set forth in (2), wherein, after said first interlayer film is removed to expose the light-shielding metal film on the top of said hill-shaped section, a second interlayer film which is thinner than said first interlayer film is formed, and thereafter the gate insulating film is formed.
Further, the present invention relates to
(4) a liquid crystal display device as set forth in one of items (1)-(3), wherein said hill-shaped section is formed so as to enclose the TFT.
Further, the present invention relates to
(5) a liquid crystal display device as set forth in one of items (1)-(3), wherein said hill-shaped section is formed on either side of a region where the semiconductor layer of the TFT is formed in the direction parallel to a gate line so that said region may become groove-shaped.
Further, the present invention relates to
(6) a liquid crystal display device as set forth in (4) or (5), wherein a portion of said semiconductor layer of the TFT constitutes a storage capacitor section and the interlayer film laid between the semiconductor layer and the underneath light-shielding film in said storage capacitor section is made thinner than in the TFT section.
Further, the present invention relates to
(7) a manufacturing method of an active matrix type liquid crystal display device having a structure in which a pixel TFT is disposed in a trench carved in a substrate; which comprises the steps of:
growing an underlying insulating film on a transparent insulating substrate;
forming a trench for disposing a pixel TFT by etching said underlying insulating film;
forming an underneath light-shielding film on an inner wall of said trench;
forming on the entire surface of the substrate, a first interlayer film to cover said underneath light-shielding film;
forming a semiconductor layer in said trench; and
forming a gate insulating film and then a metal electrode layer on said semiconductor layer; wherein
in forming said trench, a section which is not carved in but left hill-shaped is formed in the vicinity of the TFT, and the underneath light-shielding film disposed beneath the semiconductor layer of the TFT is formed so as to reach at least the top of said hill-shaped section; and the metal electrode layer formed above the semiconductor layer of the TFT extends to the top of said hill-shaped section; and which further comprises the step of:
etching away, in the direction of the film thickness, at least a part of an interlayer insulating film laid between said underneath light-shielding film and metal electrode layer so as to make a film thickness of the interlayer insulating film on the top of said hill-shaped section thinner than in other sections.
Further, the present invention relates to
(8) a manufacturing method as set forth in (7), wherein, after the first interlayer film on the top of said hill-shaped section is removed to expose the underneath light-shielding metal film, a second interlayer film which is thinner than said first interlayer film is formed over the entire surface, and thereafter the semiconductor layer is formed on said second interlayer film.
Further, the present invention relates to
(9) a manufacturing method asset for thin (7)or (8), wherein said hill-shaped section is formed so as to enclose the TFT.
Further, the present invention relates to
(10) a manufacturing method asset for thin (7) or (8), wherein said hill-shaped section is formed on either side of a region where the semiconductor layer of the TFT is formed in the direction parallel to a gate line so that said region may become groove-shaped.
Further, the present invention relates to
(11) a manufacturing method asset for thin (9) or (10), wherein a portion of said semiconductor layer of the TFT constitute a storage capacitor section, which further comprises the step of making, in the direction of the film thickness, at least a part of the first interlayer film which is included in an interlayer film laid between the semiconductor layer and the underneath light-shielding film in said storage capacitor section thinner than in the TFT section.
Further, the present invention relates to
(12) a manufacturing method as set forth in (11), wherein the step of making the first interlayer film in said storage capacitor section is carried out concurrently with etching of the top of said hill-shaped section.
In the present invention, an underneath light-shielding film extends from a trench section where a semiconductor layer of a TFT is to be disposed to the top of a hill-shaped section formed to surround the trench, and, on the top of the hill-shaped section, a film thickness of an interlayer insulating film laid between the underneath light-shielding film and a metal electrode layer formed above the semiconductor layer of the TFT is made thinner that the one in other sections thereof, especially the one of an interlayer insulating film laid between the underneath light-shielding film and the semiconductor layer, and thereby the light traveling from the reverse of the substrate such as the reflected light from an optical system can be cut off sufficiently so that the incident light coming from the reverse may not reach a channel layer of the TFT or create the leakage current, which suppresses cross-talk or the like that may lower the display quality.
In addition, an interlayer film laid between the semiconductor layer of the storage capacitor section and the underneath light-shielding film may be made thinner and the section may be made to act as a storage capacitor, whereby an increase in capacitance may be attained.